In an integrated circuit chip, signals are transferred out of the circuit by output buffers, configured as part of the integrated circuit on the chip. General purpose output buffers generally perform relatively standard functions that are useful on all outputs, such as isolation of the integrated circuit from the load it is driving and the provision of sufficient power in the driving of the output signal to maintain circuit performance, even when the load includes significant capacitance. In other words, for circuits driving loads with a high capacitance, such as a system bus, for example, the general purpose output buffer is intended to provide sufficient power to charge up that capacitance quickly enough to effectively deliver the output signal to the load within a short enough time interval considered acceptable for the integrated circuit's intended function.
The design of general purpose output buffers for a given integrated circuit is typically optimized for a single supply voltage, for example VCC=3.3 V. This is because the entire integrated circuit is generally intended to run at a given supply voltage. However, the supply voltage for a given integrated circuit may not be the same from chip to chip, because of variations in the manufacturing process for the chip. Unfortunately, the circuit performance tends to depend upon the supply voltage, so that chips having higher supply voltages perform better than necessary, while chips having lower supply voltages perform more poorly than desired. The problem may be addressed by designing the output buffer with larger transistor devices, ensuring adequate performance at supply voltages at the lowest end of the expected range of variation of the supply voltage. However, for chips having a supply voltage at higher supply voltages their output buffers will draw excessive current because of the larger transistor devices, and therefore may dissipate excessive power.